In waveguide resonators, the resonator walls influence the propagation of a laser beam and shape its mode, to a certain extent. A waveguide resonator may be defined as having a Fresnel number (NF=a2/(λL), where a is half of the resonator aperture, λ is the wavelength of the laser beam, and L is the resonator length) that is less than 0.5. One advantage of using a waveguide resonator to form a laser beam is that it allows a decrease in the transverse dimensions of the laser beam inside the waveguide. Smaller transverse dimensions permit an increase in the diffusive cooling efficiency of a laser gas bounded by the waveguide by allowing efficient transportation of waste heat to the walls of the waveguide resonator. With such efficient cooling, the plasma temperature may be lowered, thereby increasing gain, and the pressure of the laser gas may be increased. In sum, waveguide resonators may lead to higher power per volume of laser gas and faster optical response to radio frequency (RF) pulses of pump energy.
“Slab” lasers take advantage of efficient cooling in a waveguide axis of the resonator, while allowing the other axis of the resonator to behave with free-space characteristics. Power can be coupled out of the laser cavity by designing the resonator to be unstable in the free-space direction of the slab waveguide. By way of example, the resonator of the slab laser may take the form of a negative or positive branch unstable resonator (the negative branch design being more popular, for alignment stability reasons). In a negative branch unstable resonator, light is coupled out of the laser cavity by allowing recirculating energy bouncing between mirrors of the resonator to “walk off” an edge of one end mirror.
The power output of a slab laser may be increased by making the slab waveguide wider (along its free-space direction), within limits imposed by the physical structure of the laser and/or by the required quality of the beam to be emitted. The power output of a slab laser may also be increased by making the slab waveguide longer (along the length of the resonator), which may be advantageous when considering the spatial and longitudinal modes. However, simply increasing the length of a laser body may be undesirable in the market and/or prohibitively expensive with a ceramic body.